Pleochroic dyes are roughly classfied into two groups. One group dyes are those in which the direction of transition moment of visible light absorption is almost in parallel with the direction of the longer axis of the molecule and which, when dissolved as a guest molecule in a host liquid crystal, are aligned so that the alignment direction of the longer axis of the dye molecule becomes the same as that of the axis of oriented liquid crystal molecule. Such dyes are called pleochroic dyes having parallel dichroism (or P-type dyes). The other group dyes are those in which the direction of transition moment of visible light absorption is almost perpendicular to the direction of the longer axis of molecule and which, when dissolved as a guest molecule in a liquid crystal, are aligned so that the aligment direction of the longer axis of the dye molecule becomes the same as that of the axis of oriented liquid crystal molecule. Such dyes are called pleochroic dyes having perpendicular dichroism (or N-type dyes). The present invention relates to a dye belonging to the first group, i.e., a pleochroic dye having parallel dichroism, and a liquid crystal composition containing the dye.
Pleochroic dyes are chracterized in that the degree of light absorption depends upon the relative relation between the direction of absorption transition moment of dye molecule and that of electric vector of light. That is, the absorption becomes maximum when the direction of absorption transition moment is in parallel with the electric vector of light, and becomes minimum when the direction of absorption transition moment is perpendicular to the electric vector of light.
When a nematic, cholesteric or smectic liquid crystal containing such pleochroic dye is sandwiched between facing two electrodes and a voltage is applied to it across the electrodes, the liquid crystal molecules exhibit turbulent motion or are uniformly aligned along the direction of electric field depending upon the dielectric characteristics or fluid characteristics of the liquid crystal. In this occasion, the pleochroic dye molecules also move together with the liquid crystal molecules, and hence relative relation between the direction of absorption transition moment of the pleochroic dye molecule and the electric vector of incident light is changed, and consequently, the light absorption characteristics of a liquid crystal display devices are changed.
Such a phenomenon is widely known as "guesthost effect" and a color display device using electric control can be constituted by utilizing this effect (see "Guest-Host Interaction in Nematic Liquid Crystals: A New Electro-Optic Effects" reported by G. H. Heilmeier and L. A. Zanoni in Applied Physics Letters, Vol. 13, p. 91 (1968)).
For example, when a nematic liquid crystal containing a pleochroic dye showing a parallel dichroism and having a positive dielectric anisotropy is interposed between two transparent electrode plates having been subjected to homogeneous orientation treatment on each side to be in contact with the liquid crystal and facing in parallel with respect to each other, the liquid crystal molecules form a homogeneous alignment wherein the long axes of the molecules are aligned in a given direction parallel with the electrode planes (see FIG. 1). In this situation, the pleochroic dye molecules 5 dissolved in the liquid crystal are also aligned so that their long axes are also parallel with the electrode planes in the same direction. When white light 6 having travelling in a direction perpendicular to the electrode plane and which is polarized in the same direction as the alignment direction of liquid crystals by a polarizing plate 8 is transmitted through the guest-host material in the above-described alignment, its electric vector becomes parallel with the long axes of the pleochroic dye molecules, and hence a specific wavelength region is strongly absorbed by the pleochroic dye molecules. As a result, the guest-host material appears strongly colored. Then, when an electric field is applied to the liquid crystal material in the above-described alignment through the aforesaid transparent electrode plates, since the host liquid crystal has a positive dielectric anisotropy, the host liquid crystal molecules 4 and the guest pleochroic dye molecules take on a homeotropic alignment wherein the longer axes of the molecules are perpendicularly aligned with respect to the electrode planes (see FIG. 2). In this situation, since the longer axes of the pleochroic dye molecules are perpendicular to the electric vector of the incident white polarized light 6, the incident light is scarcely absorbed by the pleochroic dye molecules, and the guest-host material appears weakly colored. Thus, the formation of displays by electric driving can be realized by utilizing the above-described difference between the strongly colored state and the weakly colored state.
The pleochroic dyes to be used as a guest in a liquid crystal display utilizing the above-described guest-host effect are required to possess: (1) a high "order parameter" in a host liquid crystal; (2) a hue according to the end-use; (3) a sufficient solubility in a host liquid crystal; and (4) a high stability (light stability, heat stability, and electric stability).
Of the above-described requirements, order parameter (1) (usually presented as S) means the degree of alignment of absorption axis of dye molecule with respect to orientation direction of host liquid crystal molecules, and is defined by the folliowing equation: EQU S=1/2(3 cos.sup.2 .theta.-1)
wherein the term of cos.sup.2 .theta. is timewise averaged, and .theta. represents an angle which the absorption axis of the dye molecule makes with the orientation direction of host liquid crystal molecules. The order parameter S of pleochroic dye molecule is experimentally determined by the following equation: ##EQU1## wherein A.parallel. and A.perp. represent the absorbances of the dye molecules for the light polarized parallel to and perpendicular to the orientation direction of the host-liquid crystal, respectively.
Specifically, the order parameter S is a value which governs the contrast of a guest-host type liquid crystal display device. With pleochroic dyes having parallel dichroism, the nearer the value to 1 which is the theoretical maximum, the less the degree of residual color in white parts, which serves to realize bright and highly contrasty display.
As to the hue (2) referred to hereinbefore, the dyes must satisfy requirements for a wide variety of hues, taking into consideration the purposes of coloration such as to increase displayed information, increase degree of freedom of design, and improve fashionability. Basically, if three primary colors of yellow, magenta, and cyan are obtained, all hues can be obtained by subtractive mixture of them. Therefore, with respect to the problem of hue, the three primary colors of yellow, magenta and cyan become important.
An object of the present invention is to provide a pleochroic dye which satisfies al the requirements (1), (3) and (4) described hereinbefore and a liquid crystal composition containing the dye.
Relationship between the molecular structure of pleochroic dye and the various properties has not fully been clarified yet, and hence it is quite difficult to select a pleochroic dye which has a desired hue and satisfies all requirements described hereinbefore based on knowledges about known dyes.